A typical power management system of an electric or hybrid vehicle is shown in FIG. 1. It consists of a battery pack 101 (usually comprised of large number of Lithium-Ion or Lithium-Polymer cells), a step-up stage (usually boost-based converter) 103, and a motor drive 104 providing power for an electric motor 102. In many vehicle designs there is regenerative braking which permits recharging of the battery pack 101, the details of which are omitted for clarity in FIG. 1.
In some cases, the power management system also includes a cell balancing circuit 105, which compensates for different states of charges (SOC) of individual cells, as shown in FIG. 1. The SOC variations usually occur due to the aging and variations in the manufacturing process. Through cell balancing the effective capacity of the battery pack can be significantly increased.
The balancing circuits can be divided into two general categories. The first one is the passive balancing systems, in which the cells are balanced by dissipating energy from excessively charged cells, through resistors.
The second category is the active balancing systems, which are far more efficient. In these systems, the energy of over-charged cells is transferred to those with less charge using dc-dc converters. Even though the benefits of the active cell balancing are known, their use is relatively sparse, due to the overly large extra cost and weight the cell balancing circuits add to the system.
Balancing systems of possible interest include those described in the following US patent published applications:    US 2012-0256593 A1    US 2012-0249052 A1and in the following granted US patents:    U.S. Pat. No. 7,936,150    U.S. Pat. No. 8,269,455and in the following published international patent application:    WO/2012/172468all of which are owned by the same assignee as the assignee of the present patent application.
A prior-art balancing circuit 105 might utilize a buck-boost and a Cuk converter for cell balancing. These topologies can be implemented with a relatively small number of active components and regulated with fairly simple controllers. However, these circuits are fairly large in form factor, which reflects on the overall physical size of the system. Implementations based on the use of a bi-directional flyback and a two stage flyback converters have been proposed. Compared to other solutions, these systems lower efficiency at high power levels.
Others have proposed a configurable system for cell balancing using a large number of switches to transfer the energy between cells. The main drawback of such a system is that the balancing becomes too slow for the energy transfer between cells having similar output voltages.
Many prior-art approaches direct themselves only to a single one of the functions suggested by the functional blocks of FIG. 1. This prompts the alert reader to recall the functional block 103. The step-up converter 103 of FIG. 1 steps up the voltage from the battery 101 to the extent that is required to provide a desired DC voltage to the load 104. It will be appreciated that the step-up converter 103 as shown in FIG. 1 is required to be able to accommodate the entirety of the power transferred to the load 104. This means that the converter 103 must use switches (typically semiconductor switches) and reactive components (for example capacitors and inductors) that allow the converter to pass the full load power, rated for full load currents and rated for the full reverse voltages that might arise in serving the full load voltage. In some cases the current and voltage for which the switches must be rated is larger than the current and voltage at the output, for example in a flyback circuit. The switches in a boost converter also conduct larger than the load current.
It would be desirable if a way could be found to accomplish the aims of the functional blocks of FIG. 1 in a more integrated way, using fewer components than in prior-art approaches, and using components that would not require the full-voltage ratings of the semiconductor switches in some prior-art approaches. The cost of a semiconductor switch often increases at least linearly with the voltage rating of the switch and may increase faster than linearly. Thus there are rewards for the designer who devises topologies and approaches that permit use of components with smaller voltage ratings as compared with those needed in prior-art topologies and approaches.